1. Field of the Invention
The present invention relates generally to an evaporative type cooling system for an internal combustion engine wherein liquid coolant is permitted to boil and the vapor used as a vehicle for removing heat therefrom, and more specifically to such a system which is able to suppress pump vapor locking and similar cavitation problems and/or compensate for coolant return pump malfunction without the need to include auxiliary apparatus for said purposes.
2. Description of the Prior Art
In currently used "water cooled" internal combustion engines such as shown in FIG. 1 of the drawings, the engine coolant (liquid) is forcefully circulated by a water pump, through a cooling circuit including the engine coolant jacket and an air cooled radiator. This type of system encounters the drawback that a large volume of water is required to be circulated between the radiator and the coolant jacket in order to remove the required amount of heat. Further, due to the large mass of water inherently required, the warm-up characteristics of the engine are undesirably sluggish. For example, if the temperature difference between the inlet and discharge ports of the coolant jacket is 4 degrees, the amount of heat which 1 Kg of water may effectively remove from the engine under such conditions is 4 Kcal. Accordingly, in the case of an engine having an 1800 cc displacement (by way of example) is operated full throttle, the cooling system is required to remove approximately 4000 Kcal/h. In order to achieve this, a flow rate of 167 liter/min (viz., 4000-60.times.1/4) must be produced by the water pump. This of course undesirably consumes several horsepower.
Further, the large amount of coolant utilized in this type of system renders the possiblity of quickly changing the temperature of the coolant in a manner that instant coolant temperature can be matched with the instant set of engine operational conditions such as load and engine speed, completely out of the question.
FIG. 2 shows an arrangement disclosed in Japanese Patent Application Second Provisional Publication Sho. 57-57608. This arrangement has attempted to vaporize a liquid coolant and use the gaseous form thereof as a vehicle for removing heat from the engine. In this system the radiator 1 and the coolant jacket 2 are in constant and free communication via conduits 3, 4 whereby the coolant which condenses in the radiator 1 is returned to the coolant jacket 2 little by little under the influence of gravity.
This arrangement while eliminating the power consuming coolant circulation pump which plagues the above mentioned arragement, has suffered from the drawbacks that the radiator, depending on its position with respect to the engine proper, tends to be at least partially filled with liquid coolant. This greatly reduces the surface area via which the gaseous coolant (for example steam) can effectively release its latent heat of vaporization and accordingly condense, and thus has lacked any notable improvement in cooling efficiency.
Further, with this system in order to maintain the pressure within the coolant jacket and radiator at atmospheric level, a gas permeable water shedding filter 5 is arranged as shown, to permit the entry of air into and out of the system. However, this filter permits gaseous coolant to readily escape from the system, inducing the need for frequent topping up of the coolant level.
A further problem with this arrangement has come in that some of the air, which is sucked into the cooling system as the engine cools, tends to dissolve in the water, whereby upon start up of the engine, the dissolved air tends to come out of solution and forms small bubbles in the radiator which adhere to the walls thereof and form an insulating layer. The undissolved air also tends to collect in the upper section of the radiator and inhibit the convection-like circulation of the vapor from the cylinder block to the radiator. This of course further deteriorates the performance of the device.
Moreover, with the above disclosed arrangement the possibility of varying the coolant temperature with load is prevented by the maintainance of the internal pressure of the system constantly at atmospheric level.
European Patent Application Provisional Publication No. 0 059 423 published on Sept. 8, 1982 discloses another arrangement wherein, liquid coolant in the coolant jacket of the engine, is not forcefully circulated therein and permitted to absorb heat to the point of boiling. The gaseous coolant thus generated is adiabatically compressed in a compressor so as to raise the temperature and pressure thereof and thereafter introduced into a heat exchanger (radiator). After condensing, the coolant is temporarily stored in a reservoir and recycled back into the coolant jacket via a flow control valve.
This arrangement has suffered from the drawback that when the engine is stopped and cools down the coolant vapor condenses and induces sub-atmospheric conditions which tend to induce air to leak into the system. This air tends to be forced by the compressor along with the gaseous coolant into the radiator. Due to the difference in specific gravity, the air tends to rise in the hot environment while the coolant which has condensed moves downwardly. The air, due to this inherent tendency to rise, forms pockets of air which cause a kind of "embolism" in the radiator and which badly impair the heat exchange ability thereof. With this arrangement the provision of the compressor renders the control of the pressure prevailing in the cooling circuit for the purpose of varying the coolant boiling point with load and/or engine speed difficult.
U.S. Pat. No. 4,367,699 issued on Jan. 11, 1983 in the name of Evans (see FIG. 3 of the drawings) discloses an engine system wherein the coolant is boiled and the vapor used to remove heat from the engine. This arrangement features a separation tank 6 wherein gaseous and liquid coolant are initially separated. The liquid coolant is fed back to the cylinder block 7 under the influence of gravity while the relatively dry gaseous coolant (steam for example) is condensed in a fan cooled radiator 8.
The temperature of the radiator is controlled by selective energizations of the fan 9 which maintains a rate of condensation therein sufficient to provide a liquid seal at the bottom of the device. Condensate discharged from the radiator via the above mentioned liquid seal is collected in a small reservoir-like arrangement 10 and pumped back up to the separation tank via a small constantly energized pump 11.
This arrangement, while providing an arrangement via which air can be initially purged to some degree from the system tends to, due to the nature of the arrangement which permits said initial non-condensible matter to be forced out of the system, suffers from rapid loss of coolant when operated at relatively high altitudes. Further, once the engine cools air is relatively freely admitted back into the system. The provision of the bulky separation tank 6 also renders engine layout difficult.
Further, the rate of condensation in the consensor is controlled by a temperature sensor disposed on or in the condensor per se in a manner which holds the pressure and temperature within the system essentially constant. Accordingly, temperature variation with load is rendered impossible.
Japanese Patent Application First Provisional Publication No. sho. 56-32026 (see FIG. 4 of the drawings) discloses an arrangement wherein the structure defining the cylinder head and cylinder liners are covered in a porous layer of ceramic material 12 and wherein coolant is sprayed into the cylinder block from shower-like arrangements 13 located above the cylinder heads 14. The interior of the coolant jacket defined within the engine proper is essentially filled with gaseous coolant during engine operation at which time liquid coolant sprayed onto the ceramic layers 12.
However, this arrangement has proven totally unsatisfactory in that upon boiling of the liquid coolant absorbed into the ceramic layers, the vapor thus produced and which escapes toward and into the coolant jacket, inhibits the penetration of fresh liquid coolant into the layers and induces the situation wherein rapid overheat and thermal damage of the ceramic layers 12 and/or engine soon results. Further, this arrangement is of the closed circuit type and is plagued with air contamination and blockages in the radiator similar to the compressor equipped arrangement discussed above.
FIG. 7 shows an arrangement which is disclosed in U.S. Pat. No. 4,549,505 issued on Oct. 29, 1985 in the name of Hirano. The disclosure of this application is hereby incorporated by reference thereto.
For convenience the same numerals as used in the above mentioned Patent are also used in FIG. 7.
However, this arrangement while solving the drawbacks encountered with the previously disclosed prior art has itself suffered from the drawbacks that unless the level of coolant in the lower tank 128 is carefully maintained at the level of the level sensor 130 after the engine and cooling system have become heated to the point of becomming thermally saturated the temperature of the coolant collected in the lower tank tends to approach its boiling point. This tendancy tends to be pronounced when the engine is operated under high speed/load conditions wherein relatively large amounts of fuel are combusted and the amount of heat to be removed from the engine tends to maximize. Under these conditions upon the coolant return pump being energized to pump coolant from the lower tank to the coolant jacket, the coolant undergoes a slight depressurization in the chambers of the pump and tends to boil. This produces vapor which induces "vapor lock" or "cavitation" as it will be referred to hereinafter. Once this phenomenon occurs, control of the all important coolant level in the coolant jacket is placed in jeopardy. If the level of coolant in the coolant jacekt cannot be be maintained at the required level, the bumping and frothing of the liquid coolant therein can become so violent as to induce localized dryouts or cavitation in and around the zones where maximum heat flux occurs. This latter mentioned phenomenon also tends to induce relatively large amounts of liquid coolant to bump over into the vapor transfer conduit and lead to the situation wherein the condenser becomes partially filled with liquid coolant. This reduces the surface area of the radiator available for the coolant vapor to release its latent heat of evaporation and induces the situation wherein the boiling point of the coolant becomes excessively elevated inviting engine overheat. Simultaneously, the pump cavitation problem tends to persist as the coolant which is being bumped over from the coolant jacket enters the radiator with a temperature close to its boiling point.
One method of overcomming the pump cavitation problem is to use a small displacement capacity pump. This type of pump does not produce depressurizations of the magnitude of the larger types which induces the heated coolant to suddenly boil, but lacks the ability to return sufficient coolant to the coolant jacket under high load/high speed operational modes.
To overcome this problem it is possible to add a second coolant return pump and energize the same only when needed. However, this is provision adds unduly to the cost of the system and induces the need for additional conduiting which clutters and complicates the system.
The provision of liquid coolant traps and separators between the coolant jacket and the radiator, while preventing large volumes of the liquid coolant from entering the radiator still does not alleviate the pump cavitation problem.
Further, if the coolant return pump 136 should for some reason become inoperative to the point of not returning coolant to the coolant jacket due to a mechanical malfuction such as "sticking" of a moving part, disconnection of the pump element and the motor, or the like, level control especially in the coolant jacket becomes impossible and the system soon becomes inoperative.